Those numbers are significant because the agreement, negotiated last December, will formally “enter into force” when 55 countries, representing 55 percent of global emissions, join the effort. Thirty-one countries joined the accord at a ceremony Wednesday morning, or shortly before it, at the United Nations headquarters in New York. Those included the relatively large emitters Mexico (1.70 percent of global emissions), Argentina (0.89 percent), Ukraine (1.04 percent) and the United Arab Emirates (0.53 percent of emissions).

The pact “now brings us over the 55 countries necessary … and all that is left now to do is get the 55 percent of emissions,” said U.S. Secretary of State John F. Kerry said at the event. “But this is a great accomplishment today, and everybody should be proud of what has happened.”

U.N. Secretary General Ban Ki-moon said that he was “ever more confident that the Paris Agreement will enter into force this year,” and Kerry added that he also was “absolutely confident” of that outcome.

The ceremony also featured an airing of videos, submitted by leaders from numerous countries, pledging that they, too, would complete the necessary domestic steps that would allow them to legally join the agreement this year. A number of other relatively large emitters were on that list, including Canada (1.95 percent) and Australia (1.46 percent).

Perhaps most significant was a video from the European Union, which represents 12.10 percent of global emissions across all of its member states. If they’re all on board and complete their paperwork in conjunction with the broader union, then the agreement easily would enter into force. The E.U. is now pursuing an ultrafast ratification of the agreement that could take place as early as next month.

Once the Paris Agreement “enters into force,” the world will then proceed to operate under its auspices. Pledges by countries to reduce their emissions become formal, and the world will be committed to taking steps to limit the warming of the globe to “well below” 2 degrees Celsius above preindustrial levels, with the hope of keeping warming to 1.5 degrees C.

In the minds of world leaders racing to achieve that milestone is a tightening U.S. presidential race that could lead to the election of Republican nominee Donald Trump, who does not accept the science of climate change and has said he would “cancel” the Paris Agreement. Patricia Espinosa, the executive secretary of the U.N.’s Framework Convention on Climate Change, told Climate Wire, “I think everybody’s aware of that possibility and just concerned about it.”

Hundreds of American scientists also recently released a statement that criticizes “the Republican nominee for President” for his assertion that he would withdraw from the agreement.

If the agreement does enter into force, however, it would take four years for the United States to withdraw from the pact (unless it took the dramatic step of also abandoning the long-standing 1992 Framework Convention itself, which would take one year).

The growing likelihood that the Paris agreement could enter into force less than a year after it was negotiated represents a truly striking development, observers said.

“I don’t recall another major international agreement that entered into force within a year of being adopted,” said David Sandalow, the inaugural fellow at Columbia University’s Center on Global Energy Policy. “This is much faster than almost anyone anticipated, even during the Paris conference.”

Sandalow added that while the agreement will not officially enter into force this week during the United Nations summit in New York, the fact that more than the required 55 countries have now signed on is a significant milestone.

“It’s yet another indication of the overwhelming global consensus on the need to take action on the climate,” he said.

One White House official, who declined to speak on the record because the diplomatic process is ongoing, agreed that the conventional wisdom when the Paris agreement was signed last December was that it might take several years for it to enter into force.

But he said that change – “from a significant improbability to a near certainty” – began to take root when the United States and China jointly announced at the end of March their intention to help bring the international accord into force as soon as possible.

“There was a sense that if China and the U.S. weren’t going to accelerate entry into force, there was no chance other countries were going to put their shoulder behind this,” the official said. “It was important that we went first and were public so the conversation with the other countries could become more real and more concrete.”

And by all accounts, there have been many such conversations. Whether with India, Korea, Japan or Canada, President Obama and other officials, namely Secretary of State John Kerry, repeatedly have nudged other world leaders to official embrace the climate agreement.

“The president’s basic strategy on this has been to use every engagement he’s had … to encourage countries to move more quickly,” the official said, adding that the result shows there is reason to hope the world can solve its climate challenges. “It’s actually possible in this area of climate change to move the international community in a significant way. That should give everybody confidence that the world can do more ambitious things on climate change.”

Chris MooneyChris Mooney covers climate change, energy, and the environment. He has reported from the 2015 Paris climate negotiations, the Northwest Passage, and the Greenland ice sheet, among other locations, and has written four books about science, politics and climate change. Follow

Brady DennisBrady Dennis is a national reporter for The Washington Post, focusing on the environment and public health issues. He previously spent years covering the nation’s economy. Dennis was a finalist for the 2009 Pulitzer Prize for a series of explanatory stories about the global financial crisis. Follow